1. Field of the Invention
The present invention relates to a conveying mechanism that conveys a recording medium by means of conveying belts, and a recording apparatus including the same.
2. Description of the Related Art
An inkjet recording apparatus is an apparatus that discharges minute ink droplets from a plurality of nozzles provided in a recording head, forming an image on a recording medium. Examples of the recording head include a recording head mounted on a carriage that is movable in the width direction of the recording medium, the recording head performing recording on the entire range in the width direction of the recording medium while moving in the width direction of the recording medium, and a plurality of recording heads arranged in a line covering the entire range in the width direction of the recording medium to perform recording.
Examples of a method for conveying a recording medium in an inkjet recording apparatus include looping an endless belt (hereinafter referred to as “conveying belt”) over a plurality of rollers including a driving roller to convey a recording medium, which is brought into contact with a front surface of the conveying belt. One of the methods for enhancing the recording medium's contact with the conveying belt is an electrostatic attraction method in which charge is applied to the front surface of the conveying belt to attract a recording medium to the belt by means of an electrostatic force.
For a method for controlling a conveying belt used in a conveying belt-used conveying mechanism, controlling a conveying belt by directly detecting movements of the conveying belt itself has been proposed in, for example, Japanese Patent Application Laid-Open No. 2007-210725. In this method, a scale pattern is formed integrally with a portion of a conveying belt, or a scale member is stuck on a front or back surface of a conveying belt, and the scale pattern or member is read by means of an encoder. In such conveying mechanism, the recording medium electrostatically attracted to the conveying belt can be considered as moving together with the conveying belt. In other words, reading movements of the conveying belt can be considered as being equivalent to directly reading movements of the recording medium, enabling provision of high-precision conveyance.
For performing what is called “borderless recording”, which causes no margins in the edge portions of a recording medium, in a recording apparatus employing such conveying belt-used electrostatic attraction conveyance method, a plurality of conveying belts is arranged in the width direction of a recording medium to be conveyed (see, for example, Japanese Patent Application Laid-Open No. 2005-305688). When borderless recording is performed, ink is discharged outside a recording medium, and thus, the conveying belts are arranged so that clearances are formed around the opposite edges in the width direction of the recording medium, enabling prevention of the conveying belts from being contaminated by ink. In this method, in order to conform to various recording medium sizes, a plurality of conveying belts is arranged so as to form clearances around the opposite edges in the width direction of a recording medium of a respective size (see FIG. 4 in Japanese Patent Application Laid-Open No. 2005-305688). Consequently, the ink discharged outside the recording medium arrives at these clearances, preventing the conveying belts from being contaminated.
In conveyance of a recording medium using a plurality of conveying belts, the speeds of movement of the conveying belts may differ depending on, e.g., the differences in material among the respective conveying belts, and/or the differences in circumferential length and/or thickness among the respective conveying belts, resulting in the amounts of conveyance of the recording medium differing depending on each conveying belt. Consequently, a problem arises in that the recording medium cannot be conveyed straight. In order to solve this problem, increasing the conveyance force of a particular conveying belt from among a plurality of conveying belts to be higher than those of the other conveying belts has been proposed (see, for example, Japanese Patent Application Laid-Open No. 2007-175907). In this method, the conveying belts other than the particular conveying belt, which have a smaller conveyance force, cause slippage between the respective conveying belts and a recording medium, and the particular conveying belt having a large conveyance force conveys the recording medium, reducing the differences in the conveyance amount of the recording medium among the conveying belts. Furthermore, in order to eliminate the differences in recording medium conveyance speed among the respective conveying belts, detecting movements of the respective conveying belts and driving the conveying belts individually has been proposed (see, for example, Japanese Patent Application Laid-Open No. 2008-200972).
In order to eliminate the differences in recording medium conveyance speed among a plurality of conveying belts in the aforementioned method disclosed in Japanese Patent Application Laid-Open No. 2007-175907, it is necessary to provide a driving mechanism, a conveying belt movement detecting mechanism and a conveying belt movement adjusting mechanism for each conveying belt to adjust the driving force and movement of the conveying belt, resulting in an increase in cost and recording apparatus size. Furthermore, where none of a driving mechanism, a movement detecting mechanism and a movement adjusting mechanism is provided for each conveying belt, in order to eliminate the differences in conveyance speed among the respective conveying belts, it is necessary to provide a tension to the respective conveying belts to eliminate looseness of the conveying belts. Actual conveying belts may have an error in circumferential length depending on their materials and manufacturing methods. Accordingly, where the respective conveying belts include a material with a high elastic modulus, if all the conveying belts are looped over a same tension roller, it is difficult to provide a tension to all the conveying belts because the respective conveying belts extend/contract only to a small degree. More specifically, it is difficult to provide a tension to the conveying belt with the largest circumferential length. Therefore, a tension roller is provided for each conveying belt in addition to a plurality of rollers included in the conveying mechanism. Consequently, a tension can be provided to all the conveying belts, which, however, results in an increase in cost and apparatus size.
Where conveying belts include a material with a low elastic modulus, which can extend/contract, a tension can be provided to all the conveying belts by means of one roller. However, if conveying belts include a material with a low elastic modulus, it is difficult to directly detect the movement amounts of conveying belts to enhance the recording medium conveyance precision by means of the aforementioned method disclosed in Japanese Patent Application Laid-Open No. 2007-210725. More specifically, because the conveying belts extend/contract to a large degree, it is difficult to precisely detect the movement amounts of the conveying belts, resulting in deterioration in recording medium conveyance precision.